Pilot studies in the Human Physiology Laboratory over the past five years have sought to pinpoint a leading variable or group of variables which would predict the onset of ovulation in the human female. A promising predictor appears to be breast temperature and heat flow. The purpose of the proposed study is to explore this tentative correlation in greater depth with a more sophisticated experimental design and a larger subject population. The subjects will be monitored via radiotelemetry for a three month interval as they go about their daily lives. In addition, each subject will spend a total of eight to ten days in the laboratory where her physical and hormonal status will be further evaluated.